This invention generally relates to the technology of manufacturing optical fibers and, more particularly, it relates to an improved method of accurately and reliably measuring the tension of an optical fiber when it is being drawn.
With a typical known method of manufacturing optical fibers, the source material is molten by heat and the molten material is pulled and thinned to form a line. The optical fiber obtained by such heating and pulling may be coated with an appropriate material such as resin forthwith. The operation of producing an optical fiber by heating and pulling a source material is referred to as drawing.
In the process of coating the surface of an optical fiber as it is drawn, the tension of the optical fiber is observed immediately before and after the coating in order to check if the fiber is coated properly or not. An optical fiber that shows drawing tension differently before and after the coating, it can more often than not become broken to give rise to a troubled manufacture line. If such an accident occurs, measures have to be taken to provide and maintain an appropriate level of optical fiber drawing tension before and after the coating process typically by changing the temperature of molten resin to be used for coating the optical fiber. The tension of drawing an optical fiber also affects the light transmitting performance of the produced optical fiber. For example, if an optical fiber is produced with an excessive level of tension and coated with strain remaining in the inside, the coated optical fiber will show a large transmission loss. Therefore, it is important to observe the optical fiber drawing tension and regulate the drawing tension according to the reading of the tension from the viewpoint of producing optical fibers with little transmission loss.
The use of the oscillation waveform of an optical fiber that is being drawn as means for measuring the tension of drawing the optical fiber is already well known. With this technique, the fundamental (natural) oscillation frequency f of the optical fiber is determined from the frequency spectrum of the observed oscillation waveform and the obtained value is used as substitute for f in equation [T=(2.multidot.L.multidot.f).sup.2 .multidot..rho..multidot..alpha.] in order to determine the tension T being applied to the optical fiber, where L may represent the distance between the optical fiber source material and the coating die (first coating die) for forming a first coat layer on the optical fiber or the distance between the first coating die and the second coating die for forming a second coat layer, n represent the linear density and a represent a correction factor.
FIGS. 4A and 4B of the accompanying drawing illustrate the spectrums of respective oscillation waveforms that were actually obtained at an optical fiber manufacture line. Of these, FIG. 4A shows the frequency spectrum of an oscillation waveform with a peak frequency of 5.664063 Hz observed when an optical fiber is moved with a line speed of 100 m/min., whereas FIG. 4B shows the frequency spectrum of an oscillation waveform with a peak frequency of 7.1289006 Hz observed when an optical fiber is moved with a line speed of 980 m/min.
It will be clear by comparing FIGS. 4A and 4B that the frequency spectrum of the oscillation waveform of an optical fiber obtained at an optical fiber manufacture line contains noise components in addition to the fundamental oscillation frequency and hence it is not possible to accurately determine and regulate the tension of drawing the optical fiber from such a frequency spectrum Thus, the net result of using the known technology of coating the surface of an optical fiber while it is being drawn will be an optical fiber manufacture line that cannot effectively avoid broken optical fibers nor provide optical fibers with little transmission loss.
In view of the above identified technological problem, it is therefore the object of the present invention to provide a method of measuring the tension of drawing an optical fiber by means of the oscillation waveform of the optical fiber being drawn that can accurately, reliably and easily detect a peak frequency in the frequency spectrum of an oscillation waveform without being affected by noise.